1. Field of the Invention
The present invention relates to a stave for a metallurgical furnace such as a blast furnace.
2. Description of the Related Arts
The structure of a blast furnace wall is such that a stave (a cooling stave) including an inside cooling system is disposed inside of a shell, and on the inside of the stave, furnace refractories are held. After the blast furnace is operated for a fixed period of time, the furnace refractories are frequently dropped from the stave to be directly exposed to the inside of the furnace. Therefore, the stave must be resistant to a thermal load of inside the furnace when the furnace refractories are dropped.
Conventionally, a cast-iron stave for a blast furnace is widely used, and specifically, a cast-iron stave in which, cooling pipes are packed by insert casting, is a general structure. However, the cast-iron stave has a low cooling capacity due to a low thermal conductivity of cast-iron. Accordingly, in the bottom portion of a blast furnace (bosh portion, steeply rising portion, and bottom shaft portion) in which melted slag exists as a high thermal load region, the stave body is prone to cracking due to a high thermal stress on the stave body resulting in water leakage from cracks transferred to cooling pipes.
In order to prevent the cooling pipes from cracking, a boundary between the cooling pipes and cast-iron portion is generally made unfused; however, this results in further reduction in the cooling capacity of the stave. To make up for these disadvantages, there are some countermeasures such as increasing the cooling water, increasing the number of cooling pipes, and duplexing the body of the stave. However, these are not preferable because of complexity in the stave structure and an increase in the manufacturing cost of the stave. Even in these countermeasures, the cooling capacity is not sufficient when applied to the bottom portion of a blast furnace, which is a high thermal load region. Therefore, these problems are actualized when harsh demands are made such as an extended life of a blast furnace, and severe operating conditions by blowing-in of a large amount of pulverized coal.
On the other hand, a stave for a metallurgical furnace made of copper (or a copper alloy) is known. A copper stave has an advantage in that the inside of the furnace can always be maintained at a low temperature because of better thermal conductivity than that of cast iron. By its high cooling capacity, the following functions can be obtained, especially in a high thermal load region of a blast furnace. That is, in a high thermal load region of a blast furnace bottom portion, if furnace refractories are dropped from the stave body, melted slag is coagulated at once to form an inseparatable coagulated slag layer on the surface of the stave, when making contact with a surface of the stave. Because of a very low thermal conductivity of the inseparatable coagulated slag layer, it protects the copper-made stave from a high thermal load and also appropriately controls heat extraction from the furnace by the stave.
Therefore, in order to extend the life of a blast furnace, to depress the energy loss due to excess cooling in a high thermal load region, and to simplify the furnace wall structure to reduce cost, a copper made stave is most preferable for a blast furnace, especially for a high thermal load region thereof.
As for staves made of copper used singly for a blast furnace, conventionally known are a type of stave obtained by machining rolled or forged copper (Japanese Examined Patent Publication No. 63-56283) and a type of stave in which, cooling pipes are packed by insert copper casting. Furthermore, as for a stave made of copper used in combination with cast iron staves, a jacket-type stave is known.
However, these conventional copper staves have the following problems.
A stave made of copper obtained by machining rolled or forged copper has disadvantages such as high manufacturing cost due to complex machining and a small degree of freedom in its shape. More specifically, for example, it has the following problems.
(1) While a curvature of the stave body is necessary in accordance with the inner diameter of the furnace, it is very difficult for such a curvature to be economically formed when the stave is machined from a material such as rolled copper. Accordingly, the stave body is unavoidably designed in a flat shape, reducing an operating volume of the furnace.
(2) It is necessary to form bosses and ribs for fixing the back of the stave to a shell of the furnace. These parts must be separately machined and welded, increasing the manufacturing cost.
(3) When protrusions or grooves for holding furnace refractories and coagulated slag are formed in a cooling surface of the inside of the furnace, these must be machined from a thick plate, increasing the manufacturing cost.
(4) When new copper staves are placed in an existing furnace having cast iron staves to be used in combination with the existing cast iron staves, the thickness of the copper staves must agree with that of the cast iron stave to maintain the profile of the inside of the furnace. In this case, the thickness of the copper stave is up to 250 mm, resulting in high cost machining from a material such as rolled copper. Occasionally, such a thick material cannot be obtained.
(5) When copper staves are applied to the portion of a furnace between a belly (steeply rising portion) and a shaft, it is necessary to form the stave in an elbowed shape in a vertical direction, increasing the manufacturing cost by machining and bending.
Furthermore, in a stave made of copper obtained by machining rolled or forged copper, a path for a coolant inside of the stave must be shaped by boring. To form a corner portion of the path for a coolant, it is necessary for it to be plug welded at each one end of the holes after the holes are bored so as to be orthogonal with each other. Since the corner portion formed in this manner is L-shaped, head loss of a coolant (normally, cooling water) flowing therethrough is increased to increase energy loss. Furthermore, in this L-shaped corner portion, the cooling water stagnates and is prone to produce deposits on an inner surface of the path in this region. When these deposits successively accumulate, this increases head loss of cooling water and decreases thermal conductivity between the cooling water and the stave, reducing the cooling capacity by cooling water. Furthermore, when the the cooling water stagnates as described above, air bubbles are produced by turbulent flow of the cooling water to reduce the cooling capacity. The above-mentioned increased head loss affects the velocity of the cooling water, reducing the stave functions along with the above-mentioned reduced cooling capacity.
A stave in which cooling pipes are packed by insert copper casting has the following problems. In particular, this cannot be practically used because of problems which will be described as the following (1) to (3).
(1) Since the cooling pipe is hot deposited with the casting to insert and is at most appressed to the casting, a clearance between them is normally formed. Due to the clearance, thermal conduction is not sufficient between the cooling pipe and the casting, thereby being prone to failure of the casting due to a thermal load from the inside of the furnace. The bare cooling pipe in such a failure will deform and be subjected to wear, resulting ultimately in water leakage from the damaged pipe.
(2) The cooling pipe may be recrystallized by a casting heat to reduce the strength of the cooling pipe, resulting in breaking of the pipe.
(3) Since the melting point of the cooling pipe and that of the cast copper are the same, the cooling pipe may be dissolved and damaged, depending on the casting temperature. To avoid this, when the casting temperature is reduced, a defect caused by gas is prone to occur. To avoid this problem, when the cooling pipe is made of iron, the entire cooling capacity of the stave is reduced due to the lower thermal conductivity of iron.
(4) In order to form a path for a coolant inside the stave, it is necessary to bend the cooling pipe with a high accuracy and occasionally bend to it in a complicated shape, resulting in increased manufacturing cost.
(5) When a cooling pipe is packed by insert casting, it is difficult to accurately position the pipe to obtain the product as designed. In the bending portion of the cooling pipe especially, a curvature before casting may be increased, reducing dimension accuracy.
When a jacket-type copper-made stave used in combination with cast iron staves is supposed to be singly used, there are also the following problems.
(1) While quantity of the cooling water which can be supplied to each stave has a predetermined limit due to the capacity of a pump, because of a large cross-sectional area of a path for a coolant in a jacket-type copper-made stave, the velocity of the cooling water is inevitably reduced. In order to resist the thermal load from a furnace, the velocity of cooling water is required to be approximately 1 to 3 m/sec. In a jacket-type cast copper stave, the velocity may be up to 1 m/sec (no less than 0.3 m/sec if less than 1 m/sec, in general). This may result in damage by dissolution of the stave due to the thermal load from the furnace.
(2) In a jacket-type cast copper stave, independent multiple lines of the cooling paths will be complicated in structure. As described above, since the cross-sectional area per line is large, only a maximum of two lines can generally be provided. Therefore, when the stave is partially damaged by dissolution, there is a danger that the function of the cooling paths will be entirely lost. When the leakage of the cooling water occurs due to partial damage by dissolution, even an inspection and a repair of the leakage cannot be performed because of the danger that the stave will be completely damaged if the cooling water is stopped or reduced for the inspection and the repair.
(3) In a jacket structure, since the number of turning portions in the path for a coolant is large, a head loss of the cooling water is increased, increasing the loss of energy. It is necessary to form bosses (holes for fixing) for fixing the back of the stave to a shell of the furnace. In a jacket structure, parts of the bosses extend into the path for a coolant, impeding the flow of the cooling water, resulting in a head loss of the cooling water. In a corner portion of a jacket structure, the cooling water stagnates and is prone to produce deposits on an inner surface of the path in this region. When these deposits successively accumulate, these decrease thermal conductivity between the cooling water and the stave. Furthermore, when the cooling water stagnates as described above, air bubbles are produced by turbulent flow of the cooling water, reducing the cooling capacity by the cooling water. These problems may also cause reduction in stave functions.